Photoexcitation material and method for producing photoexcitation material

ABSTRACT

A photoexcitation material includes: a wurtzite type solid solution crystal containing gallium, zinc, nitrogen and oxygen, wherein a peak (A) of an existence ratio of nitrogen or oxygen which is a first adjacent atom of the gallium or zinc and a peak (B) of an existence ratio of gallium or zinc which is a second adjacent atom of the gallium or zinc satisfy a relational expression of A&gt;B in a relationship between a distance and the existence ratio of the adjacent atom of the gallium or zinc, the relationship being obtained from an extended X-ray absorption fine structure analysis.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2016-108751, filed on May 31,2016, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a photoexcitationmaterial and a method for producing the photoexcitation material.

BACKGROUND

Technologies utilizing solar energy include an artificial photosynthesistechnology, a photocatalytic technology and the like,

Related technologies are disclosed in, for example, Japanese Laid-OpenPatent Publication No. 2006-116415.

SUMMARY

According to one aspect of the embodiments, a photoexcitation materialincludes: a wurtzite type solid solution crystal containing gallium,zinc, nitrogen and oxygen, wherein a peak (A) of an existence ratio ofnitrogen or oxygen which is a first adjacent atom of the gallium or zincand a peak (B) of an existence ratio of gallium or zinc which is asecond adjacent atom of the gallium or zinc satisfy a relationalexpression of A>B in a relationship between a distance and the existenceratio of the adjacent atom of the gallium or zinc, the relationshipbeing obtained from an extended X-ray absorption fine structure analysis

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims. It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example of an electronic state ofGa_(x)N_(x)Zn_(1-x)O_(1-x);

FIG. 2 illustrates an example of dispersion of a conduction band ofGa_(x)N_(x)Zn_(1-x)O_(1-x);

FIG. 3 illustrates an example of a Ga₈Zn₈N₈O₈ super cell;

FIG. 4 illustrates an example of a relationship between a displacementamount and a forbidden bandwidth when positions of gallium/nitrogen andzinc/oxygen are displaced in a c-axial direction;

FIG. 5A illustrates an example of a photograph of aGa_(0.5)N_(0.5)Zn_(0.5)O_(0.5) powder;

FIG. 5B illustrates an example of a photograph of a thin film obtainedby NPD film formation of the Ga_(0.5)N_(0.5)Zn_(0.5)O_(0.5) powder;

FIG. 5C illustrates an example of a photograph of a thin film obtainedby NPD film formation of the Ga_(0.5)N_(0.5)Zn_(0.5)O_(0.5) powder;

FIG. 6A illustrates an example of a relationship between an interatomicdistance to an adjacent atom determined for Ga atoms by an extendedX-ray absorption fine structure and a coordination number;

FIG. 6B illustrates an example of the relationship between aninteratomic distance to an adjacent atom determined for Zn atoms by anextended X-ray absorption fine structure and a coordination number;

FIG. 7 illustrates an example of a variation in a peak intensity and anatomic position; and

FIG. 8 illustrates a forbidden bandwidth of a thermodynamically stablepowder and a forbidden bandwidth of a thin film, which is obtained byNPD film formation under a condition S, when x in aGa_(x)N_(x)Zn_(1-x)O_(1-x) solid solution is changed.

DESCRIPTION OF EMBODIMENTS

In an artificial photosynthetic technology, a hydrogen gas is producedfrom water and an organic matter is synthesized from water and carbondioxide. In the photocatalytic technology mentioned earlier, forexample, contaminants are decomposed. For example, photo excitationmaterials are used for these technologies,

A photoexcitation material is a semiconductor having a forbidden bandbetween a valence band and a conduction band. In the photoexcitationmaterial, electrons in the valence band are excited into the conductionband by absorbing the sunlight, and as a result, holes are generated inthe valence band. The generated excited electrons or holes reduce oroxidize water or contaminants. In order to increase the utilization ofsolar energy, a photoexcitation material may be provided which absorbs asolar spectrum from a short wavelength to a long wavelength as long aspossible. In order to provide such a photoexcitation material, an energywidth of the forbidden band may be narrow.

Gallium nitride (GaN) and zinc oxide (ZnO) are ultraviolet lightresponsive photoexcitation materials having forbidden bandwidths ofabout 3.1 eV and about 3.2 eV, respectively. Each of GaN and ZnO has awurtzite type crystal structure. When GaN and ZnO are mixed in a certainratio, a Ga_(x)N_(x)Zn_(1-x)O_(1-x) solid solution having the samecrystal structure is formed.

In the Ga_(x)N_(x)Zn_(1-x)O_(1-x) solid solution, a forbidden bandwidthis narrower than that of pure GaN and ZnO. This is because a (N 2p)-(Zn4s, 4p) bond is newly generated in the vicinity of the top of thevalence band in the Ga_(x)N_(x)Zn_(1-x)O_(1-x) solid solution. Thenarrowness of the forbidden bandwidth brings about high efficiency oflight energy use. For example, making gallium nitride (GaN) and zincoxide (ZnO) solid solution simply may have a limitation in narrowness ofthe forbidden bandwidth. For example, a photoexcitation material havinga narrow forbidden bandwidth that may utilize light energy with highefficiency may be provided.

The photoexcitation material contains gallium, zinc, nitrogen andoxygen. The photoexcitation material has a wurtzite type solid solutioncrystal. In the photoexcitation material, a peak (A) of the existenceratio of nitrogen or oxygen which is a first adjacent atom of gallium orzinc and a peak (B) of the existence ratio of gallium or zinc which is asecond adjacent atom of gallium or zinc satisfy the following equation(1) in a relationship between a distance and the existence ratio of anadjacent atom of gallium or zinc in the photoexcitation material, whichis obtained from an extended X-ray absorption fine structure analysis.

A>B   (1)

The photoexcitation material may be represented by the following formula(1),

Ga_(x)N_(x)Zn_(1.00-x)O_(1.00-x)   (1)

In the formula (1), x may fall within a range of 0.00<x<1,00,particularly, 0.25≦x≦0.75.

In the photoexcitation material, the bandgap energy may be 2.5 eV orless, and may be, for example, 2.2 eV or less. The lower limit value ofthe band gap energy is not particularly limited but may be appropriatelyselected depending on the intended purpose. For example, the band gapenergy may be 2.0 eV or more.

The photoexcitation material is a material that absorbs light in such amanner that electrons are excited,

In the Ga_(x)N_(x)Zn_(1-x)O_(1-x) solid solution, the forbiddenbandwidth is the narrowest when x is about 0.5. The forbidden bandwidth(band gap energy) at this time is about 2.5 eV. The narrowest forbiddenbandwidth that has been obtained so far in Ga_(x)N_(x)Zn_(1-x)O_(1-x) isabout 2.5 eV when x is 0.5, but the radiation with photon energy of 2.5eV or more in the solar spectrum is only about 20% of the total energyradiation. Therefore, by further narrowing the forbidden bandwidth ofGa_(x)N_(x)Zn_(1-x)O_(1-x) the utilization efficiency of sunlight may beimproved.

For example, by reducing the distance between metal ions (Ga³⁺—Ga³⁺,Zn²⁺—Zn²⁻, Ga³ ⁺—Zn²⁺) constituting a Ga_(x)N_(x)Zn_(1-x)O_(1-x)crystal, the forbidden bandwidth of Ga_(x)N_(x)Zn_(1-x)O_(1-x) may befurther narrowed.

In this case, the distance between a metal ion (Ga³⁺ or Zn²⁺) and ananion (N³⁻ or O²⁻) may not be reduced as much as possible. Therefore, aposition of the metal ion may be shifted from a position defined by anideal crystal structure. In that case, a distance between some metalions is enlarged, but a distance between some other metal ions isreduced. In the Ga_(x)N_(x)Zn_(1-x)O_(1-x) solid solution, even if theposition of the metal ion is shifted, a distance from anions surroundingthe metal ion remains substantially constant and a tetrahedronconstituted by the anions is distorted so that the positional shift ofthe metal ion is absorbed.

In order to shift the position of the metal ion in theGa_(x)N_(x)Zn_(1-x)O_(1-x) solid solution, deposition on the substrateis carried out while adding distortion to the crystal powder. To thatend, nano particle deposition (NPD) may be used.

FIG. 1 illustrates an example of an electronic state ofGa_(x)N_(x)Zn_(1-x)O_(1-x). The 4s, 4p orbital of Ga or Zn combines withthe 2p orbital of O or N to form a bond-antibond level. SinceGa_(x)N_(x)Zn_(1-x)O_(1-x) is strongly ionic, metal 4s and 4p electronsare ionized and mostly move to the 2p orbital of anion. A bond levelhaving the property of anion 2p orbital is occupied by electrons and anantibond level having the property of metal 4s and 4p orbital is anempty level. The bond level constitutes a valence band and the antibondlevel constitutes a conduction band.

FIG. 2 illustrates an example of dispersion of a conduction band ofGa_(x)N_(x)Zn_(1-x)O_(1-x). As illustrated in FIG. 2, the conductionband indicates a bonding state in which the phase of a wave functionconsisting of an antibond level is uniform among crystal sites at thecenter (Γ point) of the Brillouin zone, and has the lowest energy(conduction band bottom: CBM). At the edge of the Brillouin zone of theconduction band, wave functions are in opposite phases between sites andthe energy is the highest in a case of giving an antibonding interactionbetween sites. A difference between the highest energy and the lowestenergy corresponds to a bandwidth. By bringing a distance between sitescloser from an equilibrium distance, the interaction is strengthened toexpand the bandwidth. When the bandwidth is expanded, a forbiddenbandwidth is reduced reversely. As described above, the forbiddenbandwidth is reduced by reducing the inter-site distance forming theconduction band, for example, the distance between metal ions.

In order to reduce all of inter-metal ion distances, the size of theentire crystal may be reduced. In this case, not only an inter-metalatom distance but also a metal ion-anion distance is reduced. When ametal ion-anion distance is shortened, since a separation width of thebond-antibond level illustrated in FIG. 1, is increased and theforbidden bandwidth is expanded reversely, only the inter-metal iondistance may be brought close to each other. As the positions of metalions are shifted from the positions determined by the ideal crystalstructure, some metal ions move away from each other, whereas someapproach to each other. In this case, the conduction bandwidth becomesnarrower and the forbidden bandwidth becomes wider in a portion wherethe distance is away, but since a conduction band widened in a portionwhere the distance is dose exists in the entire crystal, the forbiddenbandwidth eventually becomes narrower.

The forbidden bandwidth of Ga_(x)N_(x)Zn_(1-x)O_(1-x) where the positionof the metal ion is shifted from the position determined by the idealcrystal structure is obtained by an ab initio density functional theory(Dm simulation. x is set to 0.5, at which the forbidden bandwidthbecomes the narrowest, and a 32-atom supercell of Ga₈Zn₈N₈O₈ is used asa calculation model.

A unit cell with a wurtzite type crystal structure is a rhombohedroncontaining two atoms in total which correspond two types of atoms(typically, cation and anion), respectively. A solid solution composedof a plurality of components, each of which contains cations or metalions and anions, may be different from the crystal from the viewpoint ofperiodicity. For example, when cations and anions constituting the solidsolution are considered to be the same, the above-described solidsolution may be referred to as a wurtzite type crystal in a broad sense.Hereinafter, the term “wurtzite type crystal structure” may be used inthis sense and the unit cell may also be referred to as a corresponding2-atom (ion) cell. Since elements constituting each unit cell aredifferent in each portion in the solid solution, two unit cells of thewurtzite type crystal structure are combined and the boundary of cellsis shifted by an integer multiple of a lattice constant to form a 4-atomrhombic pillar cell. The 4-atom rhombic pillar cell is further doubledor multiplied by 8 in a-, b- and c-axial directions, respectively, toconstruct a 32-atom supercell. The arrangement of atoms of Ga, N, Zn andO in the supercell may be a special quasi-random structure. Thedimensions of the a·b axis and the c axis are set to values actuallymeasured by X-ray diffraction (XRD) for Ga_(0.5)Zn_(0.5)N_(0.5)O_(0.5).Thereafter, while maintaining a ratio of the ab axis and the c axis,which is obtained by XRD, the size of the cell is optimized so that thetotal energy obtained by simulation becomes the smallest. Thearrangement of atoms in the cell is relaxed so that a force actingbetween the atoms, which is obtained from the simulation, issufficiently low. A structure of the supercell thus obtained isillustrated in FIG. 3. At this time, the forbidden bandwidth is 0.7 eV.This value is smaller than an experimental value (2.5 eV). For example,in the DFT simulation, the forbidden bandwidth may be calculated to besmaller than the experimental value. In the DFT simulation, although theabsolute value may be varied, the tendency to increase/decrease in theforbidden bandwidth and the electronic structures of the valence bandand the conduction band are reproduced by experimental results.

In the supercell illustrated in FIG. 3, the position of Ga or Zn isshifted. In the wurtzite type crystal structure, since each atom isrelatively easily moved in the c-axial direction, the movement is set inthe c-axial direction. When only the position of Ga or Zn is shifted inthe c-axial direction, a distance of Ga or Zn to N or O directly bondedto Ga or Zn in the c-axial direction may be greatly changed. Therefore,the N or O is also moved by the same size in the same direction. In thiscase, speaking strictly, a distance between Ga or Zn and other three Nsor Os which are directly bonded to Ga or Zn but are not present in thec-axial direction is also changed. However, since a bond between Ga orZn and N or 0 and the direction of movement of Ga or Zn form an angle ofabout 110°, the amount of change in the distance may be small,

FIG. 4 illustrates an example of a relationship between a displacementamount and a forbidden bandwidth when positions of gallium/nitrogen andzinc/oxygen are displaced in the c-axial direction. In FIG. 4, theforbidden bandwidth is changed when the positions of (a) [Ga—N] and (b)[Zn—O] are changed in the c-axial direction in the supercell of FIG. 3.Although the magnitude of an effect varies depending on a moved atomicset ([Ga—N] or [Zn—O]) and the direction of movement (either positive ornegative direction of the c axis), the forbidden bandwidth may bereduced in either case. In Ga_(0.5)Zn_(0.5)N_(0.5)O_(0.5), the positionsof metal atoms may be shifted from a position determined by a crystalstructure, thereby creating a photoexcitation material with a smallerforbidden bandwidth.

A thin film is formed by nano particle deposition (NPD). In the NPD, apowder having a diameter of about micrometer is used as a raw material.The raw material is ejected from a nozzle, together with an inertcarrier gas, in a vacuum and is deposited on a substrate. In the NPD,raw material powders collide with each other in the nozzle due to thehigh-speed flow of the carrier gas and are fractured to a nanometersize. The fractured surfaces of fractured pieces of the raw materialpowders are in a state of high surface energy with dangling bonds beingexposed. The fractured pieces are blown onto the substrate to form astrong bond with the substrate or other particles through the danglingbonds. With the NPD, without using a binder, it is possible to form afilm with a raw material of a complicated composition as it is and itmay be possible to form a porous thin film depending on the conditionsbecause the fractured pieces are deposited. Depending on the conditions,when the raw material powders collide strongly with each other or thefractured pieces strongly hit the substrate, local distortion may beintroduced into crystal lattices.

FIG. 5A illustrates an example of a photograph of aGa_(0.5)N_(0.5)Zn_(0.5)O_(0.5) powder. Each of FIGS. 5B and 5Cillustrates an example of a photograph of a thin film obtained by NPDfilm formation of the Ga_(0.5)N_(0.5)Zn_(0.5)O_(0.5) powder. A forbiddenbandwidth of Ga_(0.5)N_(0.5)Zn_(0.5)O_(0.5) may be 2.5 eV and the rawmaterial powder is in yellow (FIG. 5A). According to the measurement byXRD, a solid solution powder has a wurtzite type crystal structure. WhenNPD film formation is performed under a predetermined condition (N₂carrier gas and flow rate 14 L/min, which are hereinafter referred to asa condition N) using this solid solution powder as a raw material, ayellow thin film similar to the raw material powder is obtained (FIG.5B). For example, when the condition of NPD film formation is changed,for example, the carrier gas is changed to He (which is hereinafterreferred to as a condition S), a brown thin film is obtained (FIG. 5C).This indicates that the forbidden bandwidth is reduced. The forbiddenbandwidth of this sample measured by visible ultraviolet reflectionabsorption spectroscopy is 2.0 eV.

According to the measurement of XRD, the crystal structure of the brownthin film sample is a wurtzite type crystal structure, like the rawmaterial yellow powder and its c axis is very slightly contracted ascompared to that of the raw material yellow powder. For example, in theDFT simulation, such a degree of contraction has little influence on aband structure.

FIG. 6A illustrates an example of a relationship between an interatomicdistance to an adjacent atom and a coordination number, which isdetermined for Ga atoms by an extended X-ray absorption fine structure(EXAFS). FIGS. 6A and 6B illustrate a relationship between acoordination distance of an adjacent atom and a coordination number,which is determined for Ga atoms (FIG. 6A) or Zn atoms (FIG. 6B) by theEXAFS. In these figures, a thin line indicates a raw material yellowpowder and a thick line indicates a brown NPD film. Three to four peaksare observed in both cases, but the peak of one point several Acorresponds to N or O which is closest to Ga or Zn and the peak of about3 Å corresponds to the next closest Ga or Zn. When the raw materialpowder is NPD-deposited under the condition S, the closest N or Odistance viewed from Ga or Zn and the peak intensity are not so muchchanged. However, it can be seen from the figures that the peakintensity of the next closest Ga or Zn viewed from Ga or Zn is reduced.

EXAFS refers to a phenomenon in which wavelength dependence occurs inabsorption due to interference between photoelectrons emitted from anatom by an X-ray and photoelectrons scattered and returned at least onceby other atoms in the surroundings. Therefore, a coordination numberobtained by the analysis of EXAFS is only the distance to an atomcoordinating to a certain fixed distance and the existence ratio of theatom, but a distribution of the distance to adjacent atoms existing atrandom and the existence ratio of the adjacent atoms may not beobtained. When the distance to the adjacent atoms is varied, the widthof a peak is not widened but only the intensity of the peak isdecreased. FIG. 6A illustrates that, in the thin film formed by NPDdeposition under the condition S, O or N exists at the same distance asthe raw material powder when viewed from Ga, whereas the distancebetween Ga and Ga or the distance between Ga and Zn is varied. Thedistance of 0 or N seen from Zn and the distance of Ga or Zn seen fromZn also illustrates the same tendency as that seen from Ga (FIG. 6B).This result illustrates that Ga or Zn is varied from a positiondetermined by the crystal structure and, correspondingly, anions aremoved while keeping a bonding distance of metal-anion constant to someextent, and thus, a tetrahedron constituted by the anions is distortedor rotated.

Both of the raw material powder and the NPD thin film formed under thecondition S have a wurtzite type crystal structure and the total numberof Ga or Zn surrounding Ga is the same. Assuming that the variation in acoordination distance takes a normal distribution, a distribution of Gaor Zn in the brown NPD thin film may be estimated from the change inpeak intensity (the brown NPD film occupies 70% of the raw materialpowder) illustrated in FIGS. 6A and 6B. FIG. 7 illustrates an example ofa variation in a peak intensity and an atomic position. FIG. 7illustrates a Gaussian distribution (thin line) having the standarddeviation (α=0.14 Å) obtained from a radial distribution function in astable structure by DFT calculation and a Gaussian distribution (thickline) having the standard deviation (α=0.20 Å) whose peak intensity is70%. The integral areas of both Gaussian distributions may besubstantially equal to each other. In the stable structure (thin line),the majority of atoms has a displacement of 0.2 Å or less where the bandgap energy is not so much changed by DFT calculation. In thedistribution (thick line) estimated from the results of EXAFS, moreatoms are displaced by 0.2 Å or more. As disclosed above, a local strainis given to the crystal lattice by NPD film formation under specificconditions.

FIG. 8 illustrates an example of a forbidden bandwidth. FIG. 8illustrates a result of measurement, changing x in aGa_(x)N_(x)Zn_(1-x)O_(1-x) solid solution, on a forbidden bandwidth of araw material powder (indicated by a broken line) and a forbiddenbandwidth of a thin film (indicated by a solid line) obtained by forminga NPD film under the condition S in the same manner as described above(x=0 represents ZnO and x=1 represents GaN). A raw material powder,which is the Ga_(x)N_(x)Zn_(1-x)O_(1-x) solid solution, is prepared bymixing Ga₂O₃ and ZnO so as to have a predetermined x with a ball milland subsequently calcining the mixture in a flow of ammonia for 20hours. The forbidden bandwidth of the NPD-deposited thin film is smallerin all compositions except for x=0 and x=1 which represent puresubstances, but among them, the largest forbidden bandwidth reducingeffect is obtained in a condition where x=0.5. At this time, theforbidden bandwidth is 2.0 eV and a photoexcitation material having thenarrowest forbidden bandwidth among Ga_(x)N_(x)Zn_(1-x)O_(1-x)-basedsolid solutions is obtained.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to an illustrating of thesuperiority and inferiority of the invention. Although the embodimentsof the present invention have been described in detail, it should beunderstood that the various changes, substitutions, and alterationscould be made hereto without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A photoexcitation material comprising: a wurtzitetype solid solution crystal containing gallium, zinc, nitrogen andoxygen, wherein a peak (A) of an existence ratio of nitrogen or oxygenwhich is a first adjacent atom of the gallium or zinc and a peak (B) ofan existence ratio of gallium or zinc which is a second adjacent atom ofthe gallium or zinc satisfy a relational expression of A>B in arelationship between a distance and the existence ratio of the adjacentatom of the gallium or zinc, the relationship being obtained from anextended X-ray absorption fine structure analysis,
 2. Thephotoexcitation material according to claim 1, wherein thephotoexcitation material is represented byGa_(x)N_(x)Zn_(1.00-x)O_(1.00-x), where x falls within a range of0.00<x<1.00.
 3. The photoexcitation material according to claim 2,wherein x falls within a range of 0.255≦x≦50.75.
 4. The photoexcitationmaterial according to claim 3, wherein the photoexcitation material hasband gap energy of 2.5 eV or less.
 5. A method for producing aphotoexcitation material, the method comprising: preparing a substrate;and depositing a powder of a material over the substrate using a Hecarrier gas according to a nano particle deposition method to form athin film, wherein the thin film contains a wurtzite type solid solutioncrystal containing gallium, zinc, nitrogen and oxygen, and a peak (A) ofan existence ratio of nitrogen or oxygen which is a first adjacent atomof the gallium or zinc and a peak (B) of an existence ratio of galliumor zinc which is a second adjacent atom of the gallium or zinc satisfy arelational expression of A>B in a relationship between a distance andthe existence ratio of the adjacent atom of the gallium or zinc, therelationship being obtained from an extended X-ray absorption finestructure analysis,
 6. The method according to claim 5, wherein the Hecarrier gas is ejected at a flow rate from a nozzle, together with thepowder of the material, onto the substrate.
 7. The method according toclaim 5, wherein the thin film includes the photoexcitation materialwhich is represented by Ga_(x)N_(x)Zn_(1.00-x)O_(1.00-x), where x fallswithin a range of 0.00<x<1.00,
 8. The method according to claim 7,wherein x falls within a range of 0.25≦x≦0.75.
 9. The method accordingto claim 8, wherein the photoexcitation material has band gap energy of2.5 eV or less.